This invention relates to variable power absorption torque converters, and to the use of such torque converters in the drive train of a turbomachine such as incorporated in aircraft.
A fluid turbine machine is often utilized in aircraft either as the main propulsion engine, or as an auxiliary power unit supplying power for main engine starting, and/or power for driving the aircraft accessories. The output power of such a fluid turbine machine varies as a function of altitude, temperature, and other ambient conditions. It is many times desirable to provide a fluid coupling and torque multiplication device which is driven by the relatively constant speed, prime mover fluid turbine machine in order to match the desired variable speed and torque requirements. The fluid coupling therefore is disposed in the drive train so as to be the element which imposes the load upon the turbine machine.
The power absorption capacity (and therefore the load imposed upon the prime mover turbomachine) of a fixed geometry torque converter is determined by its size, geometry, input speed, and output speed. Therefore, the power absorption capacity of conventional fixed geometry torque converter does not vary as a function of ambient conditions as does the power output capacity of the turbomachine. The load imposed by the torque converter must not exceed the power output capacity of the turbomachine, to avoid turbine stalling. Therefore, it has been conventional practice to size the torque converter to match the minimum full output power the turbomachine produces at the least favorable ambient conditions expected to be encountered. Thus, the maximum power available from the turbomachine is usable only at this least favorable ambient condition point (called design point). At all other operating conditions, the turbomachine has additional power available which may not be absorbed and utilized by the undersized torque converter. Furthermore, a fixed geometry torque converter does not absorb constant power at all ratios of converter input and output speeds, thus further requiring torque converter undersizing at all but the most favorable speed ratio condition.
It is therefore apparent that it would be highly useful to provide a fluid coupling or torque converter in the drive train from a turbomachine operable in varying ambient conditions, wherein the torque converter has variable capacity features for obtaining maximum performance of the turbine machine at all ambient conditions. In addition to the parameters listed above which determine the power absorption capacity of a fixed geometry torque converter, variable geometry torque converters have a variable power absorption capacity which is additionally a function of mass flow rate in the torque converter as well as the directional properties of the mass flow fluid velocity. Parametric influences on power absorption capacity include the angles of the primary mass flow upon exiting the impeller, the turbine and the reactor or stator vanes, meridional flow area, impeller outlet radius, and the speed ratio of the torque converter.
One type of variable capacity torque converter utilizes variably positionable stator vanes which may be selectively pivoted relative to primary fluid flow in the torque converter to alter the direction of this primary flow when passing between the impeller and turbine of the torque converter. Change of direction of the primary flow alters the effective geometry of the converter and thus its power absorption capacity. Such previous attempts have concentrated on mechanical arrangements for individually pivoting each of the stator blades. This approach, particularly in smaller size turbine machines, has led to a high degree of mechanical complexity and attendant increase in manufacturing costs and decrease in unit reliability. Another approach has been to allow the entire set of stator vanes to rotate about the torque converter centralaxis. The torque converter capacity is varied dependent upon a direction and speed of rotation of the set of stator vanes. Again, this mechanical approach has the limitations of complexity, higher cost and lower reliability. Yet another approach, altering input speed to the torque converter is not feasible in many instances, such as a turbomachine, where it is desired that the prime mover operate at constant speed.
It is apparent, therefore, that it would be highly desirable to provide a variable capacity torque converter in which the capacity may be varied non-mechanically. Such arrangement would be particularly useful in conjuction with a turbomachine, such as the relatively small fluid turbine machine utilized as auxiliary power units for starting the main engines of an aircraft, wherein it is important that the maximum power available from the turbine machine be utilized and yet that the turbine machine never be subject to stalling.